In general, around a winch drum for use in a construction machine such as a crane, a wire rope is wound in an aligned state. This prevents the wire rope to be drawn out from a winch drum or wound up by the winch drum from being twisted or bent and makes it possible to restrict portions of the wire rope from coming into unnecessary contact with each other, as a result, enabling the wire rope to have extended life.
It is conventionally known that irregular winding occurs during lowering a suspended load. The irregular winding is a phenomenon in which the aligned state of portions of a wire rope in a winch drum is disturbed. The irregular winding during lowering a suspended load is caused by rotation of the winch drum while some factor causes a wire rope to slack.
Patent Literature 1 below discloses a technique for preventing irregular winding of a wire rope during lowering a suspended load in a winch apparatus. Specifically, Patent Literature 1 discloses an irregular-winding prevention device for preventing irregular winding of a wire rope at the start of lowering operation of a winch drum. The irregular-winding prevention device compares a real rope tension acting on the wire rope drawn out from the winch drum and an irregular-winding generating tension set in advance and restrict the maximum rotational speed of a hydraulic motor rotating the winch drum when the real rope tension is less than or equal to the irregular winding generating tension, to prevent the real rope tension from being less than or equal to the irregular winding generating tension, thereby preventing the wire rope from slacking to prevent irregular winding.
On the other hand, as a new winch apparatus different from a conventional hydraulic winch apparatus, there has been developed an electric winch apparatus having an electric motor to drive a winch drum. This electric winch apparatus also has a problem of irregular winding of a wire rope that occurs during lowering a suspended load. In particular, found is a phenomenon that there occurs a slack in a wire rope to involve the irregular winding when the lowered suspended load reaches the ground, and demanded is a technique for preventing the irregular winding of a wire rope caused upon the landing of a suspended load.
The above technique of Patent Literature 1 is a technique for preventing irregular winding caused at the start of lowering operation of a winch drum, being hard to be used for preventing irregular winding from occurring upon landing of a suspended load. In addition, the technique for preventing irregular winding recited in Patent Literature 1 is premised on control of a hydraulic motor in a hydraulic winch apparatus, thus being inapplicable to an electric winch apparatus.